Using spline-IK for bones but keeping control over individual bones
The key is to have the hook modifiers before the soft-body on the curve, so the soft-body will take the hook-deformed curve as an input:



I also subdivided the curve to add it more control points that can be hooked (but subdivision affects the soft-body sim). Without the subdivison you could control only the end-point. The hook-Empties are parented to the head. This works like this:





So you can control the curve shape with hooks now. But you can't control the same bone-chain with IK-spline and FK at the same time without dependency issues. You would have to build 3 sets of bones - IK, FK and a final set to blend between the two. Look up IK/FK blending.

Here is the blend:



Another approach that I would prefer is to rig the character the usual way without any soft-body curves etc. and to put a soft-body after the armature modifier. With the Soft-body Goal vertex group you can control which parts of model will be giggly and which not. For good performance this should be done on a low-poly skinned Mesh-deform mesh controlling the high-poly character.